Retroreflective sheeting is employed in many applications that enhance the safety of pedestrians and motorists. Many of these applications require the sheeting to have an eye pleasing or cosmetic appearance. One particularly useful type of retroreflective sheeting is cube-corner retroreflective sheeting. These types of retroreflective sheetings typically include a sheet having a generally planar front surface and an array of cube corner reflecting elements protruding from the back surface. The cube corner reflecting elements generally include trihedral structures (i.e., generally having three approximately mutually perpendicular lateral faces meeting in a single corner). In use, the retroreflector is arranged with the front surface disposed generally toward the anticipated location of the intended observers. In this orientation, light incident to the front surface enters the sheet, passes through the body of the sheet to be internally reflected by the faces of the cube corner reflecting elements so as to exit the front surface in a direction substantially toward the light source, i.e., retroreflection.
The manufacture of retroreflective cube corner element arrays is typically accomplished by employing molds primarily made by known techniques, including pin bundling and direct machining. Molds manufactured by pin bundling are made by assembling together individual pins which each have an end portion shaped with features of a cube corner reflective element. The direct machining technique, also known as ruling, involves cutting away portions of a substrate to create a pattern of grooves that intersect to form structures including cube corner elements. This grooved substrate is typically used as a master from which a series of impressions, replicas, or molds may be formed. These are typically then used as molds for retroreflective sheeting. An example of direct machining is described in U.S. Pat. No. 4,588,258 (Hoopman).
Once the mold is made, retroreflective sheetings are then typically made either by thermally embossing a plastic sheet with the grooved substrate to form a molded surface or by subsequently depositing a crosslinkable, partially polymerized resin, on a mold to be replicated which is then typically exposed to radiation, e.g., actinic light or heat, to solidify the resin. An example of such replication is described in U.S. Pat. No. 3,689,346 (Rowland).
Such manufacturing processes are typically continuous processes. For continuous manufacturing of retroreflective sheeting, a tool is generally formed from a flat originally ruled substrate, or a replica thereof, into a cylinder with one or more welding lines across the width of the sleeve. The resin composition flowing into the weld line tends to stick to the molding surface and cause objectionable seam lines and defects in the resulting sheeting. Moreover, in the step of bonding an overlay film to the array of cube corner elements, defects tend to result when the weld line aligns with an embossing protrusion on an embossing roll.
The efficiency and appearance of retroreflective sheeting can be affected by thermal or mechanical stresses, effects of resin shrinkage, removal from the mold, and the shape of the mold itself. For example, in a majority of retroreflective sheeting, seam lines can be observed across the width of the retroreflective sheeting. Because these seam lines reduce the cosmetics of the sheeting and, in some instances, impair the overall retroreflectivity of the sheeting, attempts have been made to eliminate them. For example, U.S. Pat. Nos. 5,643,400 and 5,558,740 (both to Bernard et al.) describe an apparatus and a method, respectively, for producing retroreflective sheeting, wherein at least two mold surfaces are used to generate two prism arrays which are overlapped at a leading and/or a trailing edge of each array.